Assessing the nutrient status of dairy pastures
Before applying any fertiliser, farmers need to assess which nutrients and how much of each need to be applied to correct deficiencies or to balance what has been removed by the farming system. Many factors must be taken into account before making the final decision. These will be more fully discussed in Sections 10 through 14.
However, several tests and observations can be very useful, and even essential, in assisting to make more informed fertiliser decisions. An assessment of pasture nutrient requirements should include a program of soil testing and plant tissue analysis. Other tools, such as visual paddock indicators and fertiliser test strips, are also useful indicators of nutrient requirements of pastures.
What will you find out in this section?
At the completion of this section, you should be able to:
- Correctly sample a paddock for soil testing.
- Correctly obtain a sample for plant tissue analysis.
- Assess possible nutrient deficiencies in conjunction with the visual appearance of the pasture.
- Set up fertiliser test strips to assist in identifying the fertility status of your pasture.
7.1 Soil testing
Soil testing is very useful for assessing:
- Fertiliser type and rate of nutrients required.
- Lime requirements.
- Gypsum requirements.
- Changes in soil nutrient levels over time.
However, soil testing has been a greatly underutilised tool. Although a recent survey has shown that about 78% of farmers used soil tests, only about 52% used them to determine the next season's fertiliser application. Many farmers still base their current fertiliser strategy on a soil test carried out 5 to 10 years ago! Some base it on what their neighbour is doing, some on advertisements, others on 'what Dad use to do', and some on the salesman with the most 'emotional' or 'environmentally correct' or convincing message.
How can you know what to apply, if you do not even know the current state of soil fertility on your paddock, paddocks, or entire farm? Potentially you can make some big savings on the fertiliser bill if you use soil tests to determine the current state of soil fertility.
The accuracy of any soil test depends on:
- A truly representative sample being supplied to the laboratory. Many incorrect recommendations associated with soil test results occur due to poor sampling.
- The sample being packaged correctly and transported to a reliable laboratory for a comprehensive and accurate analysis.
- Correct calibration of the chosen test methods against local or regional field trials to allow reliable interpretation.
- Basing the fertiliser, lime and gypsum recommendations on a broad range of other factors, such as pasture composition, soil type, and stocking rate.
7.2 Suggested soil sampling guidelines
The standard sampling procedures outlined below are designed to minimise the effects of soil variation and to help you collect a representative sample.
7.2.1 Timing of soil sampling
As soil nutrient levels can vary throughout the year, it is best to sample at the same time each year.
Late winter to early spring is a good time to sample because soils are reasonably consistent in soil moisture content and temperature, but consider what best suits your situation.
During summer and autumn, it is difficult to obtain a good soil sample as dry, hard soils are likely to make collection of a 10-cm-deep core difficult. Even if the dry ground can be penetrated to 10 cm, some of the core bottom may drop out of the core sampler before being deposited in the sample bag. Also, it is difficult to see the urine patches in dry pasture or on bare ground, which can dramatically alter soil test results (especially potassium levels).
Paddocks should not be sampled for at least 6 to 8 weeks after an application of phosphorus, potassium or sulphur fertiliser. Sampling after straight nitrogen fertiliser is acceptable, as soil tests don't determine the level of nitrogen in the soil.
7.2.2 Selecting areas for sampling
Ideally, it would be good to be able to soil test every paddock on the farm. A technique called nutrient mapping determines the location and concentration of a range of nutrients over the whole farm. Recent nutrient mapping research using 'every-paddock' soil testing of whole farms has indicated that, for many farms, the costs involved in this approach could be outweighed by the benefits and actual savings due to more accurate nutrient type and application rate decisions. In fact, nutrient mapping on some farms has resulted in farm fertiliser bills being reduced by up to 40%. This new technique will have positive benefits for farm profitability and the environment.
In practice, though, farmers have usually only sampled a representative paddock or set of paddocks. If restricted to a minimal number of tests, the best strategy is to identify:
- The major soil types on the farm.
- Areas with different fertiliser histories.
- Areas with a history of different but regular farm management practices, such as day paddocks, night paddocks, areas regularly cut for fodder conservation, or areas used for effluent application.
- In irrigated paddocks:
- The effects of the water quality of bores or spear points (through a water quality test).
- The effect of 'cut and fill' areas.
- Newly lasered versus older paddocks.
From these areas, select a number of representative 'monitor' paddocks to sample. These monitor paddocks can be sampled regularly over time, following the same transect, to determine if the farm soil fertility is changing. Initially, these areas may need to be sampled every 1 to 2 years while in the development stage of soil fertility and then every 2 to 3 years once the maintenance stage is reached and you are confident that your fertiliser strategies are meeting the maintenance requirements of your farm. It may then be possible to rotate the areas to be soil tested each year so that the soil fertility is monitored on other parts of the farm.
Even soil samples taken and analysed correctly can vary greatly in their ability to truly represent the area being soil tested. For example, a very wide variation can exist between paddocks and within paddocks of all the nutrients for which we test soils. Figure 7.1 shows the variation of Olsen P (estimated from Colwell P data) within one paddock that was soil tested on a gridline basis, with 20 samples taken randomly around each grid point. Each number in the figure indicates a grid point and the Olsen P around that grid point. As you can see, the Olsen P measurements ranged from 8 to 40 mg/kg.
Source: State Chemistry Laboratory (Macalister Research Farm).
Figure 7.1 Variation of Olsen P (mg/kg) within one paddock
To minimise variation within paddocks and between times of testing, transect sampling is recommended. Transect sampling means sampling the same path through the paddock each time you do a soil test. Recent experiments have shown that this technique can substantially reduce the variation in the soil test results. Permanently mark the fence posts opposite the end of each path so that future sampling can be carried out along the same line. In irregularly shaped paddocks or sections, a permanent landmark, such as a tree, fence corner or dam, can be used to identify where sampling lines cross see Figure 7.2.
- Take core samples from at least 30 sites evenly spaced along one or more straight line paths that are representative of the sampling area.
- Permanently mark the fence post or peg the path so that future sampling can be carried out along the same line.
Figure 7.2 Recommended sampling sites for transect sampling
In addition to transect sampling of representative paddocks over time, soil testing is also used to investigate specific problem areas. In these cases, take the sample from the problem area or poor section of the paddock. This is referred to as poor patch sampling. Sometimes it is helpful to take a sample from a nearby area representative of good pasture to be compared to the results from the 'poor patch' sample. If the soil test results are similar for both the poor area and the good area, then the problem may be related to some other factor, such as a trace element deficiency which can be tested via a plant tissue analysis, disease or insect pests, lack of suitable pasture species or inadequate drainage or pugging.
7.2.3 Soil sampling depth
Soil nutrient levels vary with depth and usually reduce in concentration as you go deeper. It is critical that soil cores be collected to a standard sampling depth if results are to be interpreted reliably. In southern Australia, the standard depth for pasture soil samples is 10 cm. (In New Zealand and Tasmania, the standard depth is 7.5 cm, which results in higher values for some nutrients that are concentrated near the soil surface, such as phosphorus.)
Subsurface samples (taken from a depth of 10 to 60 cm below the soil surface, in 10-cm increments) may need to be taken for such problems as:
- Poor structure.
- Sulphur deficiency.
- Subsoil pH.
- Aluminium toxicity.
Subsurface soils are usually sampled using an auger to remove soil at depth.
Proper soil sampling equipment is usually available from your local Department of Environment and Primary Industries office and from the major fertiliser companies. Soil sampling kits are also available from these sources. The errors associated with using improper sampling tools, such as spades, are large; and such sampling tools are not recommended.
7.2.4 Taking core samples
Take cores from at least 30 sites (at least 15 sites for subsoil tests) evenly spaced along one or more straight-line or zig-zag paths that are representative of the sampling area. Cores should be taken from spots of average or below-average growth. Bare ground should only be sampled if that constitutes a significant part of the paddock. Remember that you are trying to get an 'average' sample that is representative of the entire area.
Avoid obvious dung or urine patches, stock camps, stock tracks, fertiliser dump sites, recently grazed strips, and silage or hay storage areas. It is best not to sample within 20 m of fence lines, gates, troughs or trees. Remember, if 3 or 4 cores in a 30-core sample are from urine patches, it can cause the potassium soil test result to be substantially higher than it should be. Take samples from sacrifice paddocks and those that are soon to be grazed before they are grazed, so you can avoid the urine and dung patches more easily as you can see the extra growth.
Avoid growing plant material when taking cores by inserting the sampler tip between leaves and stems. However, do not remove any soil or dead plant litter from the surface before taking the core.
Remove each core carefully from the sampler, using a clean tool or fingers, and place the core in a clean container. Discard any partial cores and resample near that site.
If the 30 cores weigh more than about 1 kg, thoroughly mix the cores and then take a subsample of about 0.5 kg for mailing to the laboratory.
Transfer the cores or the subsample to a clean sample bag and label the bag with the paddock (or area) name and the number of cores taken. Fill in as much detail as possible when completing the paddock information form. This information is crucial for the person who will interpret your soil test results and formulate a fertiliser strategy for a paddock or area to ensure that the best possible recommendation can be made.
Your samples are now ready for posting to the laboratory.
7.3 Plant tissue testing
Plant tissue testing:
- Is the preferred method for diagnosing micronutrient (trace element) toxicities, deficiencies, and plant imbalances.
- Can help to determine nutrient deficiencies in animals if taken as mixed herbage sample.
- Is helpful in diagnosing nutrient levels in pasture or crop diets offered to animals.
Many field experiments have been used to verify the results of laboratory testing of soils and of plant tissue. Research has shown that using soil tests to indicate trace-element deficiencies is very inaccurate, especially on acid soils.
The same guidelines apply for plant tissue testing as for soil test sampling:
- Take plant samples from an area representative of the area being grazed.
- Use transects.
- Take the samples in late winter to spring.
- Do not take samples until about 8 weeks after the last fertiliser application.
The results will be useful only if these sampling guidelines are followed carefully.
7.3.1 Testing for plant nutritional deficiency
When sampling for plant nutritional deficiencies, take your samples from clover plants. Clovers are the pasture species most susceptible to nutrient deficiency. It is best to sample the clover that is the most dominant in the paddock. This may be white clover, sub clover, or strawberry clover. A mixture of clover species is not recommended because the various clover species have slightly different adequate levels for each nutrient and will be at different stages of maturity. Collect the leaves and petioles (stems) from about 60 white clovers or 60 strawberry clovers or 80 to 100 sub clovers (around 2 hands cupped together and filled once with clovers).
Sample the youngest fully grown leaves and their petioles of the same species of clover Figure 7.6. Post early in the week so that samples are not left to deteriorate in a post office over the weekend. Always put samples into paper bags and avoid leaving the sample for too long before posting. Do not leave the sample in a hot tractor cabin, on the ute dashboard, etc.; and refrigerate the sample if there is a delay in sending.
Figure 7.6 Section of clover to sample for plant tissue analysis
When taking tissue samples, it is vital to include as much information as possible. Describing the visual symptoms, noting which leaf is affected (youngest or oldest), and noting whether or not there is an effect on the edge (margin) of the leaf or between the veins are important. You should also outline the paddock history in terms of pasture type, soil type and previous nutrient applications.
When taking tissue samples, you should also ensure that the plant symptoms are not the result of other stresses, such as root disease, leaf disease, waterlogging, severe frost, insect attack or recent chemical application.
Tissue samples should not be taken when the plant is under a major stress, such as lack of moisture, waterlogging, frost or recent herbicide application.
Another key point is to note the amount of dry matter the plant is producing relative to what is considered adequate (in other words, is the plant growing slowly and producing very little dry matter, or is it producing close to maximum production). This will help the person interpreting the results.
Sampling the correct plant part is also vital, as described above. However, no matter which plant part is taken, ensure you tell the laboratory which plant part you have taken. In most cases, the youngest fully open leaf is the best plant part to take, regardless of the plant type, and the sample should be taken preflowering if possible. If there is a good and bad section in a paddock or farm, often taking a sample from each area is very useful for comparison.
Supplying the species of plant, plant part and stage of growth will allow the person interpreting the results to put the correct nutrient standards in for your sample. Actual adequate levels for any nutrient vary depending on species, plant part and stage of growth, so this information is critical.
7.3.2 Testing for animal health problems
When sampling for animal health issues, collect a mixture (grasses and clovers) of the plants that the stock are eating. Include weeds if relevant. Collect at least 20 handfuls across the pasture, using scissors to cut the sample off (to avoid soil contamination from pulling). This is referred to as a mixed herbage sample.
7.4 Soil and plant tissue testing laboratories
A number of laboratories offer a soil and plant testing service to Victorian primary producers. Many of the larger laboratories have membership in the Australasian Soil and Plant Analysis Council (ASPAC), but only a limited number have membership of the National Association of Testing Authorities (NATA).
ASPAC conducts regular National Quality Assurance Programs to enhance standards of analysis and assist standardisation of soil and plant analytical methods across laboratories. Those laboratories that produce analysis results within the stringent criteria for the series of soil analyses will be provided with a 'Certificate of Proficiency' for each test in which they successfully participated. It is important to remember that ASPAC provides a proficiency certificate for individual tests, not on a whole-of-laboratory basis. Other tests that a laboratory carries out may not have been deemed sufficiently accurate to be worthy of a proficiency certificate. Ask the lab if they are accredited for the tests that you require, or check on the ASPAC website (www.aspac-australasia.com).
ASPAC offers assistance to improve laboratory procedures. It also encourages laboratories to:
- Use standard units for reporting so that results are comparable between laboratories.
- Regularly check their testing techniques by testing samples with known values.
- Use accredited methods that have been calibrated against field trials under local conditions.
NATA is an association that, among other duties, sets and maintains the high standard for the methodology of laboratory practices and technical advice and accredits laboratories.
When choosing a laboratory for soil testing or plant tissue analysis, you should consider asking:
- Have they got ASPAC accreditation for all the tests they offer or at least the tests you need?
- Has their laboratory got NATA accreditation?
- Have the test methods used been field calibrated in Victoria for pastures? (Just because a test method has been used overseas or on another crop does not mean that it is valid for interpreting Victorian pastures.)
- What is the cost?
- What is the turnaround time?
- Is the laboratory independent or is it owned by a fertiliser company?
- What is the quality of advice likely to be?
The cost of analysis per sample can be very different between laboratories due to such factors as differences in services, tests offered or the scale of the operation.
You should use the same laboratory each time you test so that the results can be compared. Using different laboratories may lead to inconsistent results. However, if you are dissatisfied with the service provided by a laboratory, you can use another one the next time you test. Just remember that the results may be less comparable with the previous test.
7.5 Visual symptoms of nutrient deficiencies in pastures
To help determine your fertiliser needs, an important step to take in conjunction with soil testing is to visually assess the pastures. The main features to look for are overall colour, clover density and leaf size, and the presence of weeds and poor pasture species. If symptoms are apparent on individual plants, then pasture production will have been below its maximum potential well before this stage. In fact, visual symptoms will not become apparent until the reduced growth has exceeded 30% (referred to as 'hidden hunger').
7.5.1 Identifying plant disorders from visual symptoms
The visual symptoms plants exhibit in response to nutritional disorders can be a useful guide for identifying the cause of a disorder. Common plant responses include unusual colours or patterns in the leaves, burns, distortion of individual plant parts, stunting or abnormal growth.
Several non-nutritional disorders Table 7.1 can also produce similar symptoms, so careful observation is needed to ensure the diagnosis is reliable.
Table 7.1 Major causes of visual symptoms in plants
|Type of Disorder||Causes|
Infectious diseases: fungal, bacterial or viral
Physiological: environmental stresses
Chemical injury: pesticide, air pollution, spray burn
In pasture plants, nutrients move from the roots to other parts of the plant through a network of cells called the vascular system. These cells specialise in moving water, nutrients and metabolic products throughout the plant. The arrangement of veins and the ease with which individual elements move within the plant (in other words, their mobility) have a strong influence on the way symptoms develop.
The close relationship between the symptom pattern and the arrangement of the veins is an important feature of nutritional disorders that distinguishes them from most symptoms of non-nutritional disorders. Non-nutritional disorders usually show no relationship to vein pattern.
7.5.2 Characteristics of nutritional disorder symptoms on leaves
Nutritional disorders produce characteristic symptoms in leaves. These include:
- Symptoms are restricted initially to a single leaf-age class, that is, young, old or intermediate-aged leaves.
- Patterns are symmetrical and closely related to leaf venation.
- Changes in leaf colour and tissue death develop gradually (rarely overnight).
- The boundaries between green and chlorotic (yellow) or necrotic (dead) areas on a symptom leaf tend to be diffuse (fuzzy or blurred). Strong, definite boundaries are often produced by herbicides or viruses.
- Leaf symptom patterns due to a nutrient deficiency are rarely blocky or angular. Such patterns can be caused by a pathogen or occasionally by nematodes.
- Nutritional problems impair cell function and rarely cause mechanical disruption of the cuticle (outer layer) of the leaf. Thus, damage to the surface of a symptom leaf is not likely to be caused by a nutritional disorder.
- Symptoms develop first in tissues most distant from the major veins of the leaf, such as the interveinal regions and the tips and edges of the leaf blade.
Visible changes in a crop, such as yellowing, small leaves and poor seedset, all begin as a breakdown in cell functioning and tell us that a nutritional disorder exists. For example, the distortion of new tissues or flowers or the death of growing points is typical of boron deficiency. These visual symptoms occur because boron is necessary for the proper regulation of cell division. Similarly, the leaves of nitrogen- or magnesium-deficient plants are pale because nitrogen and magnesium are constituents of chlorophyll.
Such links between an element's physiological function and a specific abnormality that results when it is deficient are common in plants. For this reason, the nature of the symptom can provide a useful guide to the identity of a nutritional disorder even in unfamiliar crops.
The two most important diagnostic features of a nutritional disorder symptom are:
- Where the symptom is found on the plant (location).
- Its appearance (colour and pattern).
Nutritional symptoms generally develop irregularly over a plant but show first in specific organs, such as the leaves, roots, shoots or growing points. Depending on the mobility of the element, leaf symptoms can occur in the upper, middle or lower sections of a plant.
Mobile elements like nitrogen, magnesium or potassium are moved about the plant relatively easily to satisfy local shortages, particularly in new shoots or developing seeds. When one of these mobile elements is deficient, the older leaves are the first to be depleted and the first to show symptoms.
Less mobile elements, such as iron, copper, boron or calcium, do not move readily from older to younger tissues; so when these elements are deficient, the symptoms appear in the newer or upper leaves or in the flowers or the seed.
Symptoms of nutrient toxicity generally show first in the oldest leaves. These leaves have the highest transpiration rates and receive most of the nutrients absorbed by roots as the nutrients move in the transpiration stream.
Observe the size and shape of the plant, the overall foliage colour, the colour of symptom leaves and the pattern of chlorotic (pale or yellow) or necrotic (burnt, appears dead) areas in relation to vein pattern. Also note any irregular shape, splitting, cracking or corkiness of affected organs. All of these may help to establish the identity of the disorder.
7.5.3 Deficiency and toxicity
Are the symptoms indicative of a deficiency or toxicity?
Deficiency symptoms typically occur on a single leaf-age class unless more than one problem exists.
Toxicity symptoms often develop rapidly. When this happens, the affected leaf tissue may change from healthy green to grey-green or dark brown without a transitional yellow phase.
Symptoms that appear on old and new leaves at the same time may indicate toxicity. For example, when an excess of one element causes a nutrient imbalance, deficiency symptoms may be seen in the young leaves while older leaves may show burn or other symptoms of toxicity. Excess phosphorus, manganese or zinc can cause iron deficiency chlorosis in young leaves as well as symptoms of nutrient excess (toxicity) in the old leaves.
Diagnostic keys Table 7.2 provide a framework for a visual diagnosis, but there are two major weaknesses:
- A disorder is usually quite advanced before clear visual symptoms appear, and some loss of yield or quality will have occurred. Also, the absence of symptoms in a crop or pasture does not mean that nutrition is adequate. 'Hidden hunger' is the condition in which performance is limited, but no symptoms have been expressed.
- Visual symptoms can be unreliable when more than one element is limiting or when some environmental stress has modified the normal pattern.
Table 7.2 Quick guide to nutrient deficiencies: what to look for
|Symptoms first seen in older leaves||Leaf colouration even over whole leaf|
Nitrogen: Pale-green to yellow leaves.
Phosphorus: Leaves dull, lacking lustre, bluish-green or purple colours. Poor growth.
|Leaf colouration forms a definite pattern|
Potassium: Scorching and yellowing, commonly around the edges of leaves, which may become cupped.
Magnesium: Patchy yellowing often with a triangle of green remaining at the leaf base. Sometimes brilliant red to orange patterns or scorching.
|Symptoms first seen in young leaves||Leaf colouration forms a pattern|
Sulphur: Small, pale, yellow-green leaves with lighter-coloured veins.
Iron: Almost total loss of green between veins, leaving faint green 'skeleton' of veins on leaf.
Zinc: Severe restriction of leaf size or stem length, or both (hence the terms 'little leaf' or 'rosetting'). Distinct interveinal creamy yellow patches on leaves in many species.
Copper: Tip leaves cupped, narrow, distorted or scorched. Defoliation from tip. Chlorosis interveinal or irregular.
|Symptoms first seen in either old or young leaves||Leaf colouration forms a pattern|
Manganese: Mottled diffuse pale-green to yellow patches between veins. No restriction of leaf size (unlike zinc).
|Symptoms usually most prominent in other tissues; seen first in youngest tissues and fruit||Calcium: Breakdown of parts of fruit in some species. Collapse of flower stalk (flax, rapeseed) or leaf petiole (clover).|
Boron: Internal cracking or breakdown of root or stem tissues. Irregular shaped tissues, corkiness or surface cracking of stems. Irregular flower development or poor seed set.
Source: Adapted from Weir and Cresswell (1994).
Section 6 covers all the nutrient disorder symptoms of individual nutrients in more detail.
7.5.4 General paddock symptoms
As soil fertility declines, the grasses and clovers become patchy and stunted. Gradually, weeds start to fill the gaps.
Dandelion, rib weed, white daisy, etc. are collectively called 'flat' weeds. They are regularly associated with a reduction in soil fertility, usually potassium deficiency but also phosphorus and molybdenum. This is especially evident in regular silage and hay paddocks or where insufficient fertiliser has been applied in the past.
Onion weed is an indicator of soils that are deficient in phosphorus.
Sorrel and moss are usually associated with low-pH (strongly acidic) soils but can also be acting as a filler species like those mentioned above, that is, coming into a pasture to fill in the areas vacated by the more productive grasses and clovers.
A good indicator of whether a pasture may respond to extra fertiliser is to examine the areas around the dung and urine patches. If there are healthy ryegrass and clover plants within these areas, but such plants are sparse or less healthy in the areas between the patches, then this pasture is indicating that 'If you feed me (N, P, or K), then I'll grow.'
Soils becoming saline undergo a changing of species as the level of salinity increases over time. The initial changes are a decline in white clover and an increase in strawberry clover. Then buck's horn plantain, toad rush, and windmill grass begin to invade. Yellow buttons, sea barley grass and annual beard grass indicate advanced stages of salinity.
Damage may also be caused by insects, such as lucerne flea and red-legged earth mite, and by various viruses.
Fertiliser test strips are useful for determining what nutrients to apply but are less useful for determining the appropriate application rate. Test strips may be used to check results of soil tests or as a cheap way to test soil fertility. They may be of limited use on high-fertility pastures where there are no obvious nutrient limitations to plant growth. To determine the application rate, it is often more important to prepare a budget and evaluate the costs of nutrients.
There are basically two ways to set up test strips: small hand-spread strips (20 m x 2 m) or longer machine-spread strips the length of a paddock. Whichever system is used, stock need to be kept off the strips for a period of time (at least 4 to 8 weeks, depending on the season) to allow the effects of the fertilisers to be seen.
A 20-m x 2-m test strip is equal to 1/250th of a hectare. Therefore, to apply the equivalent of 250 kg/ha, you need to weigh out 1 kg of the product to be applied. To ensure an even spread over the hand-spread strips, split the required amount of fertiliser in half and go over the plots twice.
Small hand-spread strips are easier and less costly to set up and, provided they are around 20 m in length, will cover sufficient good and bad areas of pasture to allow a comparison to be made. Several sets of test strips may be needed around the farm to help in determining the final fertiliser strategy.
7.6.1 Site selection
- Test strips are best sited towards the centre of a paddock or at least 3 metres from a fence line. Run the strips at right angles to the fence line.
- Choose an area that is typical of the paddock and, if possible, a pasture with some clover present.
- Avoid fence lines, trees, gates, stock troughs, haystacks, old firebreaks, corners of paddocks, stock camps or poorly drained areas.
- Strips will be of more use if they are put on an area that has not been top dressed that year. Alternatively, top dress the strips after the paddock has been top dressed and evaluate the potential for additional response above what is to be gained from the paddock topdressing.
- Run strips up and down a slope, rather than across it. Surface runoff immediately after topdressing can shift fertiliser from one strip to another.
7.6.2 The best time to set up a test strip
Strips can be set up at any time from March to the end of July. The best time is 3 to 4 weeks after the autumn break. By then it is possible to see if the proposed site is representative of the paddock and contains some clover.
It is also valuable to check the test strips in the following autumn approximately 3 weeks after the 'break'. This will give a good guide to any longer-term effect on pasture species and on plant growth response.
7.6.3 Creating a test strip treatment plan
Table 7.3 shows a set of typical fertiliser treatments with the amounts of fertiliser suitable for 20-m x 2-m strips. A suitable layout for this typical set of test strip treatments is shown in the site plan Figure 7.7. Additional strips could be included to look at the responses to other fertiliser mixes, such as SuPerfect Potash 3 & 1, Super M 18, reactive rock phosphate, DAP, urea or Pasturebooster.
Fowl manure, lime or nitrogen could be spread across all the treatments at either end of the strips to look at these responses. However, surface-applied lime may not show a response for a year or two, except where the lime has triggered a molybdenum response. If you apply lime, always have a molybdenum strip somewhere on the site.
Table 7.3 Typical fertiliser strip treatments
|Strip||Approximate Nutrient Rates/ha||Amount of Fertiliser/Plot||Equivalent Fertiliser Rate/ha|
|50 kg phosphorus /ha and 3.75 kg sulphur/ha||1 kg triple superphosphate||250 kg/ha|
|50 kg phosphorus/ha and 61 kg sulphur/ha||2.2 kg single superphosphate||555 kg/ha|
|100 kg potassium/ha||0.8 kg of muriate of potash||200 kg/ha|
|Control with no fertiliser||No fertiliser|
|50 kg phosphorus /ha,
100 kg potassium/ha plus 61 kg sulphur/ha
|2.2 kg single super plus
0.8 of muriate of potash
|555 kg super/ha plus 200 kg potash/ha|
|50 kg phosphorus /ha,
100 kg potassium/ha plus 6 kg sulphur/ha
plus trace elements
|2.2 kg single super plus
0.8 kg muriate of potash
plus trace elements (Cu and Mo)
|555 kg super/ha,|
200 kg potash/ha plus
3 kg trace element/ha
Figure 7.7 Typical fertiliser test strip site plan
7.6.4 Setting up the test strips
Measure out the 20-m x 2-m plots with a piece of string or tape measure, and mark the corners with semi-permanent pegs or steel posts. The edges of the strips should be marked out with string.
Draw a plan of the site, similar to Figure 7.7 above, and identify the strips by tagging the pegs before the fertiliser is spread.
When spreading the fertiliser, make sure it covers the entire width of the strip. After the fertiliser has been spread, the string can be removed and permanent pegs can be used to mark the corners of each strip.
7.6.5 Weighing and spreading the fertiliser
Weigh the fertiliser treatments for each strip, and place them in labelled bags. Before you start spreading, place the bags of fertiliser on the marked-out strips, according to the site plan.
Mix the contents of the bag in a bucket before spreading. To get an even spread of fertiliser it is best to go over the plot two or three times, using one-half or one-third of the fertiliser each time. Walk along the strip and evenly spread the fertiliser by hand, right up to the edges of the strip. Repeat going back the other way. Start off the spreading lightly so you don't run out before you reach the end of the strip. It may also pay to choose a day when it is not too windy to avoid spreading onto the adjacent strip.
If you have small quantities of fertiliser or trace elements, mix them with sand or sawdust to increase the volume being spread.
7.6.6 Assessment of the strips
The control (no fertiliser) strip is the most important. Without this, it is impossible to compare treatments to determine whether the fertiliser has had any effect or not.
When comparing the strips consider:
- Pasture height and density.
- Size and colour of clover leaves.
- Botanical composition.
- Evenness of pasture.
The strips should be regularly checked throughout the year and observations recorded, such as regrowth after grazing. The final assessment of the site will be made in spring before the grass seed heads emerge. If the paddock is going to be grazed, then the test strips will need to be fenced off. To see the effects of the fertiliser, it is important to keep stock off because they will preferentially eat the good strips where there is a response, giving the observer the incorrect answer for responses on the site.
Sometimes it may be appropriate to graze the strips off and allow them to grow again to evaluate the regrowth.
If nitrogen is applied to any strip, then evaluate it regularly from 2 weeks after topdressing. If the paddock is locked up for early silage, a temporary fence will not be required in spring when the site is assessed.
The strips can be inspected in the following years to observe carryover effects on pasture production and changes to botanical composition. The benefits of some fertilisers may not appear until the clover content of the pasture has increased, so sometimes responses are not evident until the second year. If the strips are to be observed in the second year, make sure that the test site is grazed down similarly to the rest of the paddock over summer. Remove the fences to allow better grazing and to reduce the likelihood of stock camping on the plots.
7.6.7 Interpreting the results
If there are clear differences in pasture growth between strips, you will be able to assess which nutrient or nutrients you require to improve pasture production. A 20% or greater difference in growth rate can be visually detected, whereas a pasture meter can detect about a 10% difference.
If there is poor growth on all strips, it may be due to other factors, such as poor soil structure, soil acidity, plant diseases, pests, waterlogging, salinity or lack of productive pasture species. Usually these factors have all become evident before the test strips were even established.
In areas of reasonable soil fertility, fertiliser test strips may indicate that no fertiliser is needed. In fact, even though a test strip has shown nil response, paddocks may actually respond to fertiliser application. An experiment at DEPI Ellinbank compared paddock application versus strip application of superphosphate. Although the test strips indicated that fertiliser was not required, the whole-paddock application did result in an increase in pasture production and animal gain! The reasons for response in the paddock as against this lack of visible response in the test strip paddocks are probably due to:
- The greater availability of recycled P.
- An interaction between recycled N and K with higher soil P levels.
- Selective grazing of test strips, thereby retarding regrowth due to lower pasture height.
- A pasture's nutrient requirements should be assessed using a number of methods, including soil and plant tissue testing, visual paddock indicators, and soil test strips.
- Soil and plant tissue sampling guidelines should be followed carefully to achieve the most accurate results from your tests.
- Plant tissue testing should be used to determine the need for trace elements.
- Visual assessment of pasture condition can be used in conjunction with soil testing to help determine fertiliser needs.
- Fertiliser test strips are useful for determining which fertilisers to apply but are of little value for determining the appropriate application rate.